Compression moulding dimensions

Note The injection/compression moulding process capability maps 1, 2 and 3 are used for large parts with a major dimension greater than 50 mm typically and/or for large production volumes. Map 4 is for injection moulded parts that have a major dimension less than 150 mm and which are produced in small volumes. 

With compression moulding, to ensure dimensional consistency, it is necessary to allow the excess material to move away from the edge of the cavity so that the lands between the cavities can contact with minimum thickness of rubber (flash) between them. Spew grooves and channels are provided of sufficient dimensions to accommodate this excess, and also to allow the escape of air from the mould cavity. In some cases, where the shape is complex, it may be necessary to provide extra venting to allow air to escape from a blind area, where it is likely to be trapped. 

Dimension tolerances are important in compression moulding. The dimensions of the finished article are smaller than those of the mould cavity, in particular because not only thermal shrinkage but also shrinkage during the reaction counts. When using resins which develop volatiles upon curing, extra shrinkage occurs due to the escape of these components. 

Test specimens were cut from compression moulded sheet in the form of rectangular bars with the following dimensions (a) 10 x 10 x 110 mm, (b) 16 xl6 x 110 mm and (c) 20 x 20 X 110 mm (Figure 1). A lathe was used to produce the symmetrical notch using a single point cutting tool of tip radius < 20 im. 

Compositions of polypropylene with unmodified and modified chalk were obtained by means of a screw extruder at 210 C. The granulated product was used to prepare compression moulded films 0.4 mm thick which were pressed at 190 C and then quenched in iced water. From films obtained in this way oarshaped samples with the dimensions as in Fig. 2 were punched. In all cases the samples were cut out form the same places in compression moulded films in order to provide approximately the same flow-induced orientation of chalk particles relative to the sample axis. For the purpose of tensile properties and volume changes measurements the samples were marked (see Fig. 2). 

After drying for 15 h in vacuo, polyurethanes were compression moulded at temperatures between 100 and 200 C and at a nominal load of either 8 or 12 tons. Biomer" was solvent cast in 3 layers directly from the 30% dimethylacetamide solution supplied and the sheets were dried at 40 C in a flow of dry nitrogen for 7 days. The flat sheets had dimensions of 60 mm x 100 mm and were 1 mm thick. They were cut into several pieces or punched into dumbbells 3 cm in length. The straight (testing) area of the dumbbell was 13 nun x 4 mm. All samples showed no birefringence under cross polarizers, indicating that there was no detectable residual stress. 

On a small scale, compression moulding, where the requisite quantity of polymer pellets is introduced into a hot mould of the desired dimensions, then pressed into a homogeneous mould-sized sheet, has been found satisfactory. 

From all materials, plates with dimension of 125x60x0.2 mm were compression-moulded. Material... 

Residual strains in injection moulded Nylon-6 have been measured but the stress distribution is parabolic, with a compressive stress as much as 6 Mn m on the surface and a tensile one in the middle of 4 Mn m dimension in water reverses this distribution making it tensile on the surface. This has been attributed to recrystallization in presence of water and not simple to adsorption and volume changes. The yield and fracture toughness of the Nylon-6 is affected by water content, moulding conditions due to changes in yield stress. There is a brittle to ductile transition with temperature associated with cc-transition. The yield stress increases linearly with crystallinity, whereas the fracture toughness falls consistent with a move from plane strain to plane stress conditions. 

After homogenization of the mixtures of selected particular compositions, powders were then axially compressed with the load of 110 MPa (Fig. 19) and dried in laboratory dryer. As a result of the procedure the moulded pieces were obtained with nominal dimensions of ( )=30mm, h=5mm. 

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